This invention relates to radiation generator apparatuses for medical equipment, and more particularly to the output monitoring circuits for the radiation generator apparatuses.
FIG. 1 is a block diagram showing the organization of a conventional radiation generator device, which is disclosed, for example, in Japanese Laid-Open Patent (Kokai) No. 55-133800. In FIG. 1, the circuit for an electron gun 6 and an accelerator 7 of the radiation generator apparatus includes: a pulse trigger generating circuit 1; a high-voltage pulse modulator 2 coupled to the pulse trigger generating circuit 1; a pulse transformer 3 for the electron gun 6 coupled to the high-voltage pulse modulator 2; a pulse transformer 4 coupled to the high-voltage pulse modulator 2; a high-power microwave generator 5 for the accelerator 7 coupled to the pulse transformer 4. The electron beam generated by the electron gun 6 via the output of the pulse transformer 3 is accelerated within the accelerator 7 via the microwave generated by the high-power microwave generator 5, and is deflected by a deflecting system 8 to hit on a x-ray radiation generating mechanism 9, to generate x-ray radiation.
The radiation detected by a radiation monitor or detector 10 is converted into a corresponding voltage via an amplifier 11 and is supplied to an output stabilizer circuit 12 together with the output of an output setting mechanism 13. The output of the detector 10 is also supplied via the amplifier 11 to a radiation output intensity display 20 and an integrator circuit 21, the output of which is displayed by a radiation amount display 22; and to an interlocking circuit including: a divider circuit 14, an upper bound setter 15, a lower bound setter 16, a comparator 17, another comparator 18, an AND circuit 19, an integral radiation setting mechanism 23 for setting the integral or cumulative amount of radiation, a comparator 24 for determining whether the output of the integrator circuit 21 has exceeded the value set via the integral radiation setting mechanism 23, a time setting mechanism 25 for setting the maximum duration of radiation generation, a switch mechanism 26 for effecting on/off of the radiation generation, a timer mechanism 27 for measuring the time beginning with the radiation generation, a time display 28 for displaying the duration of radiation generation, a comparator 29, an AND circuit 30, another AND circuit 31, a control unit 32 for controlling the on/off of radiation generation, and a reset mechanism 33 which, although the connection to other parts are not shown, resets the displays 22 and 28 and, if the values of the previous radiation generation are held in the integrator circuit 21 and the time display 28, the integrator circuit 21 and the time display 28, at the start of radiation generation.
The comparator 24 outputs a logical "1" when the output of the integrator circuit 21 is not greater than the value that is set via the integral radiation setting mechanism 23, and outputs a logical "0" when the output of the integrator circuit 21 exceeds the set value. The comparator 29 outputs a logical "1" when the output of the timer mechanism 27 is not greater than the value set via the time setting mechanism 25, and outputs a logical "0" when the output of the timer mechanism 27 exceeds the set value. The switch mechanism 26 outputs logical "1" upon commencement of radiation generation, to start the timer mechanism 27. The AND circuit 30 outputs the AND or logical product of the outputs of the comparator 24, the comparator 29 and the switch mechanism 26.
The comparator 17 outputs a logical "1" when the output of the divider circuit 14 is less than the value set via the upper bound setter 15, and otherwise a logical "0". The comparator 18 outputs a logical "1" when the output of the divider circuit 14 is greater than the value set via the lower bound setter 16, and otherwise a logical "0". The AND circuit 19 outputs the AND or logical product of the outputs of the comparators 17 and 18. The ON gate of the control unit 32 is opened (i.e., turned on) via the output of the AND circuit 31 taking the logical product of the outputs of the AND circuit 19 and the AND circuit 30.
FIG. 2 shows the waveform of the pulse current signal generated by the detector of FIG. 1. When the radiation from the x-ray radiation generating mechanism 9 hits on the detector 10, an ionization current having the waveform shown in FIG. 2 is generated, where: .tau..sub.W represents pulse width; i.sub.p represents peak value; and .tau..sub.R represents the pulse repetition period.
The amplifier 11 integrates the pulse current signal of FIG. 2 and converts it into corresponding voltage signal. The temporal average value I.sub.ave of the pulse waveform of FIG. 2 is represented by: EQU I.sub.ave =i.sub.P .multidot..tau..sub.W /.tau..sub.R ( 1)
Usually, not the instantaneous but the temporal average rate of the output radiation intensity must be monitored. Thus, the waveform of FIG. 2 is fed into the integrator circuit of FIG. 3 having an integration time constant equal to RC. The average output V.sub.Oave of the circuit of FIG. 3 is represented by: EQU V.sub.Oave =-R.multidot.I.sub.ave ( 2)
However, since the input to the circuit of FIG. 3 has a pulse waveform of FIG. 2, the transient or temporal variation of the output of the circuit of FIG. 3 is represented by: EQU V.sub.O (t)=-R.multidot.i.sub.P .multidot.C.multidot.e.sup.- (t-.tau.W)/RC(3)
provided that EQU .tau..sub.W &lt;&lt;RC.multidot.
Thus, the output of the circuit of FIG. 3 fluctuates with a ripple .DELTA.V, the value of which becomes greater as the repetition period of the pulse signal increases.
The above radiation generator apparatus thus has the following disadvantage.
The radiation monitoring output of the circuit of FIG. 3 is utilized, not only for the observation of the output power, but also for the output stabilization and smoothing control and for the interlocking safety circuit. Thus, when the ripple .DELTA.V becomes great, the operation of the control circuit may become unstable due to hunting or the operation of the interlocking circuit may fail. Further, the monitoring meters are hard to read due to hunting of the readings.